Apparatus and method capable of a high fundamental acoustic resonance frequency and a wide resonance-free frequency range

ABSTRACT

An embodiment of the present invention provides an apparatus capable of a high fundamental acoustic resonance frequency, comprising a substrate, a bottom electrode layer adjacent the substrate, a voltage tunable dielectric layer adjacent the bottom electrode layer, the voltage tunable dielectric layer including an active region, a top electrode adjacent the voltage tunable dielectric layer, a final interconnect layer connected to the top electrode via an interlayer, and wherein the top and bottom electrodes are at a predetermined thickness such that a desired high fundamental acoustic resonance is obtained. The active region of the voltage tunable dielectric layer may be approximately the length of the top electrode and the interlayer and the final interconnect layer may cover only a small fraction of the active region of the voltage tunable dielectric layer, thereby reducing the amplitude of resonances due to the interlayer or final interconnect layer.

BACKGROUND OF THE INVENTION

Varactors are voltage tunable capacitors in which the capacitance isdependent on a voltage applied thereto. Although not limited in thisrespect, this property has applications in electrically tuning radiofrequency (RF) circuits, such as filters, phase shifters, and so on. Themost commonly used varactor is semiconductor diode varactor, which hasthe advantages of high tunability and low tuning voltage, but sufferslow Q, low power handling capability, and limited capacitance range. Anew type of varactor is a ferroelectric varactor in which thecapacitance is tuned by varying the dielectric constant of aferroelectric material by changing the bias voltage. Ferroelectricvaractors have high Q, high power handling capacity, and highcapacitance range.

One ferroelectric varactor is disclosed in U.S. Pat. No. 5,640,042entitled “Thin Film Ferroelectric Varactor” by Thomas E. Koscica et al.That patent discloses a planar ferroelectric varactor, which includes acarrier substrate layer, a high temperature superconducting metalliclayer deposited on the substrate, a lattice matching, a thin filmferroelectric layer deposited on the metallic layer, and a plurality ofmetallic conductors disposed on the ferroelectric layer and in contactwith radio frequency (RF) transmission lines in tuning devices. Anothertunable capacitor using a ferroelectric element in combination with asuperconducting element is disclosed in U.S. Pat. No. 5,721,194. Tunablevaractors that utilize a ferroelectric layer, and various devices thatinclude such varactors are also disclosed in U.S. Pat. No. 6,531,936,entitled “Voltage Tunable Varactors And Tunable Devices Including SuchVaractors,” filed Oct. 15, 1999, and assigned to the same assignee asthe present invention.

A major concern in tunable capacitors including voltage tunabledielectric capacitors is the elimination of losses due to theelectrostrictive effect and acoustic resonances. These resonancestypically manifest as a series of bumps and ripples on the Q vs.Frequency characteristic of a tunable capacitor causing the Q-factor todip as low as 10 to 20.

A model of dielectric losses due to the electrostrictive effect andacoustic resonances has been proposed with Electrostrictive resonancesin Ba0.7Sr0.3.TiO3 thin films at microwave frequencies which uses verythin BST layers, which would ensure a sufficiently high fundamentalfrequency of the acoustic resonance—i.e. beyond the frequency range ofthe application.

However, the aforementioned thin BST layers are impractical due tomanufacturability and linearity considerations. Hence, what is needed isan apparatus and method capable of a high fundamental acoustic resonancefrequency and a wide resonance-free frequency range using films that arereadily manufacturable.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides an apparatus capable ofa high fundamental acoustic resonance frequency, comprising a substrate,a bottom electrode layer adjacent the substrate, a voltage tunabledielectric layer adjacent the bottom electrode layer, the voltagetunable dielectric layer including an active region, a top electrodeadjacent the voltage tunable dielectric layer, a final interconnectlayer connected to the top electrode via an interlayer, and wherein thetop and bottom electrodes are at a predetermined thickness such that adesired high fundamental acoustic resonance is obtained. The activeregion of the voltage tunable dielectric layer may be approximately thelength of the top electrode and the interlayer and the finalinterconnect layer may cover only a small fraction of the active regionof the voltage tunable dielectric layer, thereby reducing the amplitudeof resonances due to the interlayer or final interconnect layer.

In an embodiment of the present invention, the substrate may be chosento have a high acoustic loss factor thereby reducing the amplitude ofresonances due to the substrate layer and the voltage tunable dielectriclayer may be an approximately 300 nm thick BST layer matched with anapproximately 150 nm gold top electrode and approximately 200 nmplatinum bottom electrode and the interlayer and final interconnectlayers may cover only a small percentage of the active region.

Another embodiment of the present invention further provides anapparatus capable of a wide resonance-free frequency range comprising, asubstrate, a bottom electrode layer adjacent the substrate, a voltagetunable dielectric layer adjacent the bottom electrode layer, thevoltage tunable dielectric layer including an active region, a topelectrode adjacent the voltage tunable dielectric layer, a finalinterconnect layer connected to the top electrode via an interlayer, andwherein the top electrode is made sufficiently thick such that thefundamental acoustic resonance lies below a desired frequency range andthe bottom electrode thickness is selected to suppress the secondovertone of the acoustic resonance thereby creating a wideresonance-free frequency range lying between the fundamental and thirdovertone of the acoustic resonance. In this embodiment the active regionof the voltage tunable dielectric layer may be approximately the lengthof the top electrode and the interlayer and the final interconnect layermay cover only a small fraction of the active region, thereby reducingthe amplitude of resonances due to the interlayer or final interconnectlayer. The voltage tunable dielectric layer may be an approximately0.711 μm thick BST layer and may be matched with an approximately 0.49μm gold top electrode and an approximately 0.56 μm platinum bottomelectrode.

Yet another embodiment of the present invention provides a method ofmodeling electrostrictive effects and acoustic resonances in a tunablecapacitor, comprising adjusting empirically the characteristicimpedances and complex propagation constants to account for actualprocess variations in manufacturing of the tunable capacitor, adjustingempirically the characteristic impedances and complex propagationconstants to account for end-effects in the directions transversal tothe wave direction, modeling the electrostrictive effect by dividing aBST layer of the tunable capacitor into thin layers or slices with eachslice's thickness representing a small fraction of an acousticwavelength and injecting at each junction between slices, an acoustic“current” proportional to the electrical current through the voltagetunable capacitor. The real part of the vector sum of acoustic“voltages” at all slice junctions may be divided by the electricalcurrent vector, and may be taken to be representative of that part ofthe voltage tunable capacitor's effective series resistance contributedby the electrostrictive effect and acoustic resonances. Further, theelectrostrictive effect may be the transducer mechanism that links theelectrical and acoustic domains.

Yet another embodiment of the present invention provides a method ofproducing a high fundamental acoustic resonance frequency, comprising,placing a bottom electrode layer adjacent a substrate with a voltagetunable dielectric layer adjacent the bottom electrode layer, thevoltage tunable dielectric layer including an active region, placing atop electrode adjacent the voltage tunable dielectric layer with a finalinterconnect layer connected to the top electrode via an interlayer; andusing the top and bottom electrodes at a predetermined thickness suchthat a desired high fundamental acoustic resonance is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 illustrates a cross-sectional view of a tunable capacitorstructure of one embodiment of the present invention;

FIG. 2 illustrates a schematic diagram of one embodiment of the presentinvention;

FIGS. 3 a and 3 b illustrate a simulated relative ESR (a), and measuredESR (b), for a 300 nm thick BST layer matched with a 150 nm gold topelectrode and 200 nm platinum bottom electrode of one embodiment of thepresent invention; and

FIG. 4 depicts the measured ESR for a 710 nm thick BST layer matchedwith a 490 nm gold top electrode and 560 nm platinum bottom electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

An embodiment of the present invention provides a method of modeling theelectrostrictive effect and acoustic resonances that facilitates thedesign of arbitrary circuits in the acoustic domain of the PTCstructure. Provided herein is a first exemplary acoustic circuit designinvolving BST layers of practical thickness with metal layers arrangedsuch that a sufficiently high fundamental frequency of the acousticresonance can be achieved. It is understood that this circuit is merelyillustrative and the present invention is not limited to any particularcircuits nor should the present invention be considered to be limited toonly BST.

As another illustrative example, and not by way of limitation, anembodiment of the present invention further provides a second specificacoustic circuit design involving BST layers of practical thickness withmetal layers arranged such that a wide resonance-free frequency rangecan be selected in design.

A method of modeling electrostrictive effect and acoustic resonancesthat facilitates the design of arbitrary circuits in the acoustic domainof the PTC structure may include the following:

Step 1. It is assumed that an electrostrictive effect causes a smallamount of RF energy to be converted into acoustic energy, which existswithin the entire PTC structure as bulk acoustic waves traveling in thetwo directions perpendicular to the BST layer. These waves are modeledas equivalent electrical waves existing in transmission lines withcharacteristic impedances and complex propagation constants typicallyderived.

Step 2. The characteristic impedances and complex propagation constantsare adjusted empirically to account for actual process variations inmanufacturing of the tunable capacitors;

Step 3. The characteristic impedances and complex propagation constantsare adjusted empirically to account for end-effects in the directionstransversal to the wave direction, viz. layers extending further thanthe active area (such as the substrate) and layers not completelycovering the entire active area (such as the final interconnect layer).

Step 4. The electrostrictive effect, i.e. the transducer mechanism thatlinks the electrical and acoustic domains, is modeled by dividing theBST layer into thin layers or slices—each slice's thickness representinga small fraction of an acoustic wavelength—and injecting at eachjunction between slices, an acoustic “current” proportional to theelectrical current through the tunable capacitor. The real part of thevector sum of acoustic “voltages” at all slice junctions, divided by theelectrical current vector, is taken to be representative of(proportional to) that part of the tunable capacitor effective seriesresistance (ESR) contributed by the electrostrictive effect and acousticresonances. This method of modeling recognizes the fact that the BSTlayer has insignificant thickness in terms of electrical wavelengths,while having appreciable thickness in terms of acoustic wavelengths.

Table 1 illustrates the range of acoustic parameters that were found toprovide repeatable modeling of the ESR for various PTC designs, althoughit is understood that the present invention is not limited in thisrespect nor limited to the below values, parameters or materials. TABLE1 Ranges of Acoustic Parameters used in Modeling of ESR Min Max MaterialParameter Units value value Comment MgO Velocity of sound m/s 6,00010,000 Substrate Characteristic Ohm 13,000 35,000 Substrate impedanceAttenuation per dB 0 5 Substrate wavelength Al₂O₃ Velocity of sound m/s8,000 12,000 Substrate Characteristic Ohm 17,000 45,000 Substrateimpedance Attenuation per dB 0 5 Substrate wavelength Pt Velocity ofsound m/s 3,000 4,000 Electrode Characteristic Ohm 65,000 75,000Electrode impedance Attenuation per dB 0 2 Electrode wavelength BSTVelocity of sound m/s 4,000 10,000 Active layer Characteristic Ohm40,000 60,000 Active layer impedance Attenuation per dB 0 2 Active layerwavelength Ti Velocity of sound m/s 4,000 5,000 InterlayerCharacteristic Ohm 4,000 70,000 Interlayer impedance Attenuation per dB0 2 Interlayer wavelength Au Velocity of sound m/s 2,500 3,000Interconnect Characteristic Ohm 14,000 70,000 Interconnect impedanceAttenuation per dB 0 2 Interconnect wavelength

An embodiment of the present invention provides designing for a highfundamental acoustic resonance frequency which may include the BST beingchosen to be of a practical thickness in terms of considerations otherthan acoustic resonance. These considerations could be and are notlimited to linearity and manufacturability.

Turning now to the figures, FIG. 1, shown generally as 100, is across-sectional view of a tunable capacitor structure of one embodimentof the present invention. The top 120 and bottom 135 electrodes may bemade sufficiently thin and residing on substrate 140 such that asufficiently high fundamental acoustic resonance is obtained. As anoptional further refinement when applicable, the tunable capacitor maybe designed such that the interlayer 115 and final interconnect layer110 cover only a small fraction of the active region 130 of BST layer125, thereby reducing the amplitude of resonances due to the interlayer115 or final interconnect layer 110. The interlayer 115 and finalinterconnect layer 110 thickness and material properties may thereforebe unimportant. As an optional further refinement when applicable, thesubstrate may be chosen to have a high acoustic loss factor, Al₂O₃ beinga better choice than MgO, thereby reducing the amplitude of resonancesdue to the substrate layer 140. The substrate layer 140 thickness isunimportant. In a preferred embodiment of this design, a 300 nm thickBST layer was matched with a 150 nm gold top electrode and 200 nmplatinum bottom electrode. As further refinements, an Al₂O₃ substratewas used and the interlayer and final interconnect layers covered only asmall percentage of the active region.

Turning now to FIG. 2, generally at 200, is a schematic diagram of oneembodiment of the present invention. As can be seen from FIG. 2ESR∝Re(V₂/V₁), with, V₂/shown as 210 and V₁ shown at 205. IllustrativeBST slices of circuit 200 are shown at 220, 225, 230, 235, 240 and 245with subtrate at 217 and bottom electrode depicted at 215 and topelectrode at 250 adjacent interlayer 255 which in turn is adjacent finalinterconnect layer 260. Again, it is understood that any circuitsprovided herein are merely illustrative and the present invention is notintended to be limited to any particular circuits configurations.

FIGS. 3 (a) 305 and (b) 310 shows graphs of the ESR obtainedrespectively through simulation and measurement in Frequency in GHz vs.relative ESR (a) and Frequency in GHz vs ESR in ohms (b).

An embodiment of the present invention provides designing for a wideresonance-free frequency range where the BST may be chosen to be of apractical thickness in terms of considerations other than acousticresonance. These considerations could be and are not limited tolinearity and manufacturability. The top electrode is made sufficientlythick such that the fundamental acoustic resonance lies below thefrequency range of interest for the application. The bottom electrodemay be carefully selected to suppress the second overtone of theacoustic resonance thereby creating a wide resonance-free frequencyrange lying between the fundamental and third overtone of the acousticresonance. As an optional further refinement when applicable, thetunable capacitor may be designed such that the interlayer and finalinterconnect layer cover only a small fraction of the active region (asshown in FIG. 1), thereby reducing the amplitude of resonances due tothe interlayer or final interconnect layer. The interlayer and finalinterconnect layer thickness and material properties are thereforeunimportant. As an optional further refinement when applicable, thesubstrate may be chosen to have a high acoustic loss factor, Al₂O₃ beinga better choice than MgO, thereby reducing the amplitude of resonancesdue to the substrate layer. The substrate layer thickness isunimportant.

In an embodiment of this design, a 0.71 μm thick BST layer was matchedwith a 0.49 cm gold top electrode and 0.56 μm platinum bottom electrode.As further refinements, an MgO substrate may be used and the interlayerand final interconnect layers may cover only a small percentage of theactive region.

Turning now to FIG. 4 shows measurements of the ESR obtained with themeasured ESR for a 710 nm thick BST layer matched with a 490 nm gold topelectrode and 560 nm platinum bottom electrode. It is understood thatthese specific values are for thoroughness of description and are notmeant in anyway to limit the present invention to any particularspecific measurements.

Throughout the aforementioned description, BST has been used as atunable dielectric material that may be used in a tunable dielectriccapacitor of the present invention. However, the assignee of the presentinvention, Paratek Microwave, Inc. has developed and continues todevelop tunable dielectric materials that may be utilized in embodimentsof the present invention and thus the present invention is not limitedto using BST material. This family of tunable dielectric materials maybe referred to as Parascan®.

The term Parascan® as used herein is a trademarked term indicating atunable dielectric material developed by the assignee of the presentinvention. Parascan® tunable dielectric materials have been described inseveral patents. Barium strontium titanate (BaTiO3—SrTiO3), alsoreferred to as BSTO, is used for its high dielectric constant(200-6,000) and large change in dielectric constant with applied voltage(25-75 percent with a field of 2 Volts/micron). Tunable dielectricmaterials including barium strontium titanate are disclosed in U.S. Pat.No. 5,312,790 to Sengupta, et al. entitled “Ceramic FerroelectricMaterial”; U.S. Pat. No. 5,427,988 by Sengupta, et al. entitled “CeramicFerroelectric Composite Material—BSTO—MgO”; U.S. Pat. No. 5,486,491 toSengupta, et al. entitled “Ceramic Ferroelectric CompositeMaterial—BSTO—ZrO2”; U.S. Pat. No. 5,635,434 by Sengupta, et al.entitled “Ceramic Ferroelectric Composite Material—BSTO-Magnesium BasedCompound”; U.S. Pat. No. 5,830,591 by Sengupta, et al. entitled“Multilayered Ferroelectric Composite Waveguides”; U.S. Pat. No.5,846,893 by Sengupta, et al. entitled “Thin Film FerroelectricComposites and Method of Making”; U.S. Pat. No. 5,766,697 by Sengupta,et al. entitled “Method of Making Thin Film Composites”; U.S. Pat. No.5,693,429 by Sengupta, et al. entitled “Electronically Graded MultilayerFerroelectric Composites”; U.S. Pat. No. 5,635,433 by Sengupta entitled“Ceramic Ferroelectric Composite Material BSTO—ZnO”; U.S. Pat. No.6,074,971 by Chiu et al. entitled “Ceramic Ferroelectric CompositeMaterials with Enhanced Electronic Properties BSTO Mg BasedCompound-Rare Earth Oxide”. These patents are incorporated herein byreference. The materials shown in these patents, especially BSTO—MgOcomposites, show low dielectric loss and high tunability. Tunability isdefined as the fractional change in the dielectric constant with appliedvoltage.

Barium strontium titanate of the formula BaxSr1-xTiO3 is a preferredelectronically tunable dielectric material due to its favorable tuningcharacteristics, low Curie temperatures and low microwave lossproperties. In the formula BaxSr1-xTiO3, x can be any value from 0 to 1,preferably from about 0.15 to about 0.6. More preferably, x is from 0.3to 0.6.

Other electronically tunable dielectric materials may be used partiallyor entirely in place of barium strontium titanate. An example isBaxCa1-xTiO3, where x is in a range from about 0.2 to about 0.8,preferably from about 0.4 to about 0.6. Additional electronicallytunable ferroelectrics include PbxZr1-xTiO3 (PZT) where x ranges fromabout 0.0 to about 1.0, PbxZr1-xSrTiO3 where x ranges from about 0.05 toabout 0.4, KTaxNb1-xO3 where x ranges from about 0.0 to about 1.0, leadlanthanum zirconium titanate (PLZT), PbTiO3, BaCaZrTiO3, NaNO3, KNbO3,LiNbO3, LiTaO3, PbNb2O6, PbTa2O6, KSr(NbO3) and NaBa2(NbO3)5 KH2PO4, andmixtures and compositions thereof. Also, these materials can be combinedwith low loss dielectric materials, such as magnesium oxide (MgO),aluminum oxide (Al2O3), and zirconium oxide (ZrO2), and/or withadditional doping elements, such as manganese (MN), iron (Fe), andtungsten (W), or with other alkali earth metal oxides (i.e. calciumoxide, etc.), transition metal oxides, silicates, niobates, tantalates,aluminates, zirconnates, and titanates to further reduce the dielectricloss.

In addition, the following U.S. patent applications, assigned to theassignee of this application, disclose additional examples of tunabledielectric materials: U.S. application Ser. No. 09/594,837 filed Jun.15, 2000, entitled “Electronically Tunable Ceramic Materials IncludingTunable Dielectric and Metal Silicate Phases”; U.S. application Ser. No.09/768,690 filed Jan. 24, 2001, entitled “Electronically Tunable,Low-Loss Ceramic Materials Including a Tunable Dielectric Phase andMultiple Metal Oxide Phases”; U.S. application Ser. No. 09/882,605 filedJun. 15, 2001, entitled “Electronically Tunable Dielectric CompositeThick Films And Methods Of Making Same”; U.S. application Ser. No.09/834,327 filed Apr. 13, 2001, entitled “Strain-Relieved TunableDielectric Thin Films”; and U.S. Provisional Application Ser. No.60/295,046 filed Jun. 1, 2001 entitled “Tunable Dielectric CompositionsIncluding Low Loss Glass Frits”. These patent applications areincorporated herein by reference.

The tunable dielectric materials can also be combined with one or morenon-tunable dielectric materials. The non-tunable phase(s) may includeMgO, MgAl2O4, MgTiO3, Mg2SiO4, CaSiO3, MgSrZrTiO6, CaTiO3, Al2O3, SiO2and/or other metal silicates such as BaSiO3 and SrSiO3. The non-tunabledielectric phases may be any combination of the above, e.g., MgOcombined with MgTiO3, MgO combined with MgSrZrTiO6, MgO combined withMg2SiO4, MgO combined with Mg2SiO4, Mg2SiO4 combined with CaTiO3 and thelike.

Additional minor additives in amounts of from about 0.1 to about 5weight percent can be added to the composites to additionally improvethe electronic properties of the films. These minor additives includeoxides such as zirconnates, tannates, rare earths, niobates andtantalates. For example, the minor additives may include CaZrO3, BaZrO3,SrZrO3, BaSnO3, CaSnO3, MgSnO3, Bi2O 3/2SnO2, Nd2O3, Pr7O11, Yb2O3,Ho2O3, La2O3, MgNb2O6, SrNb2O6, BaNb2O6, MgTa2O6, BaTa2O6 and Ta2O3.

Thick films of tunable dielectric composites may comprise Ba1-xSrxTiO3,where x is from 0.3 to 0.7 in combination with at least one non-tunabledielectric phase selected from MgO, MgTiO3, MgZrO3, MgSrZrTiO6, Mg2SiO4,CaSiO3, MgAl2O4, CaTiO3, Al₂O₃, SiO2, BaSiO3 and SrSiO3. Thesecompositions can be BSTO and one of these components, or two or more ofthese components in quantities from 0.25 weight percent to 80 weightpercent with BSTO weight ratios of 99.75 weight percent to 20 weightpercent.

The electronically tunable materials may also include at least one metalsilicate phase. The metal silicates may include metals from Group 2A ofthe Periodic Table, i.e., Be, Mg, Ca, Sr, Ba and Ra, preferably Mg, Ca,Sr and Ba. Preferred metal silicates include Mg2SiO4, CaSiO3, BaSiO3 andSrSiO3. In addition to Group 2A metals, the present metal silicates mayinclude metals from Group 1A, i.e., Li, Na, K, Rb, Cs and Fr, preferablyLi, Na and K. For example, such metal silicates may include sodiumsilicates such as Na2SiO3 and NaSiO3—5H₂O, and lithium-containingsilicates such as LiAlSiO4, Li2SiO3 and Li4SiO4. Metals from Groups 3A,4A and some transition metals of the Periodic Table may also be suitableconstituents of the metal silicate phase. Additional metal silicates mayinclude Al2Si2O7, ZrSiO4, KalSi3O8, NaAlSi3O8, CaAl2Si2O8, CaMgSi2O6,BaTiSi3O9 and Zn2SiO4. The above tunable materials can be tuned at roomtemperature by controlling an electric field that is applied across thematerials.

In addition to the electronically tunable dielectric phase, theelectronically tunable materials can include at least two additionalmetal oxide phases. The additional metal oxides may include metals fromGroup 2A of the Periodic Table, i.e., Mg, Ca, Sr, Ba, Be and Ra,preferably Mg, Ca, Sr and Ba. The additional metal oxides may alsoinclude metals from Group 1A, i.e., Li, Na, K, Rb, Cs and Fr, preferablyLi, Na and K. Metals from other Groups of the Periodic Table may also besuitable constituents of the metal oxide phases. For example, refractorymetals such as Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta and W may be used.Furthermore, metals such as Al, Si, Sn, Pb and Bi may be used. Inaddition, the metal oxide phases may comprise rare earth metals such asSc, Y, La, Ce, Pr, Nd and the like.

The additional metal oxides may include, for example, zirconnates,silicates, titanates, aluminates, stannates, niobates, tantalates andrare earth oxides. Preferred additional metal oxides include Mg2SiO4,MgO, CaTiO3, MgZrSrTiO6, MgTiO3, MgAl2O4, WO3, SnTiO4, ZrTiO4, CaSiO3,CaSnO3, CaWO4, CaZrO3, MgTa2O6, MgZrO3, MnO2, PbO, Bi2O3 and La2O3.Particularly preferred additional metal oxides include Mg2SiO4, MgO,CaTiO3, MgZrSrTiO6, MgTiO3, MgAl2O4, MgTa2O6 and MgZrO3.

The additional metal oxide phases are typically present in total amountsof from about 1 to about 80 weight percent of the material, preferablyfrom about 3 to about 65 weight percent, and more preferably from about5 to about 60 weight percent. In one preferred embodiment, theadditional metal oxides comprise from about 10 to about 50 total weightpercent of the material. The individual amount of each additional metaloxide may be adjusted to provide the desired properties. Where twoadditional metal oxides are used, their weight ratios may vary, forexample, from about 1:100 to about 100:1, typically from about 1:10 toabout 10:1 or from about 1:5 to about 5:1. Although metal oxides intotal amounts of from 1 to 80 weight percent are typically used, smalleradditive amounts of from 0.01 to 1 weight percent may be used for someapplications.

The additional metal oxide phases can include at least two Mg-containingcompounds. In addition to the multiple Mg-containing compounds, thematerial may optionally include Mg-free compounds, for example, oxidesof metals selected from Si, Ca, Zr, Ti, Al and/or rare earths.

While the present invention has been described in terms of what are atpresent believed to be its preferred embodiments, those skilled in theart will recognize that various modifications to the discloseembodiments can be made without departing from the scope of theinvention as defined by the following claims.

1. An apparatus capable of a high fundamental acoustic resonancefrequency, comprising: a substrate; a bottom electrode layer adjacentsaid substrate; a voltage tunable dielectric layer adjacent said bottomelectrode layer, said voltage tunable dielectric layer including anactive region; a top electrode adjacent said voltage tunable dielectriclayer; a final interconnect layer connected to said top electrode via aninterlayer; and wherein said top and bottom electrodes are at apredetermined thickness such that a desired high fundamental acousticresonance is obtained.
 2. The apparatus of claim 1, wherein said activeregion of said voltage tunable dielectric layer is approximately thelength of said top electrode.
 3. The apparatus of claim 1, wherein saidinterlayer and said final interconnect layer cover only a small fractionof said active region of said voltage tunable dielectric layer, therebyreducing the amplitude of resonances due to the interlayer or finalinterconnect layer.
 4. The apparatus of claim 1, wherein said substrateis chosen to have a high acoustic loss factor thereby reducing theamplitude of resonances due to the substrate layer.
 5. The apparatus ofclaim 1, wherein said voltage tunable dielectric layer is 300 nm thickBST layer matched with a 150 nm gold top electrode and 200 nm platinumbottom electrode and said interlayer and final interconnect layers coveronly a small percentage of said active region.
 6. An apparatus capableof a wide resonance-free frequency range comprising: a substrate; abottom electrode layer adjacent said substrate; a voltage tunabledielectric layer adjacent said bottom electrode layer, said voltagetunable dielectric layer including an active region; a top electrodeadjacent said voltage tunable dielectric layer; a final interconnectlayer connected to said top electrode via an interlayer; and whereinsaid top electrode is made sufficiently thick such that the fundamentalacoustic resonance lies below a desired frequency range and the bottomelectrode thickness is selected to suppress the second overtone of theacoustic resonance thereby creating a wide resonance-free frequencyrange lying between the fundamental and third overtone of the acousticresonance.
 7. The apparatus of claim 6, wherein said active region ofsaid voltage tunable dielectric layer is approximately the length ofsaid top electrode.
 8. The apparatus of claim 6, wherein said interlayerand said final interconnect layer cover only a small fraction of saidactive region, thereby reducing the amplitude of resonances due to theinterlayer or final interconnect layer.
 9. The apparatus of claim 6,wherein said substrate has a high acoustic loss factor, thereby reducingthe amplitude of resonances due to the substrate layer.
 10. Theapparatus of claim 6, wherein said voltage tunable dielectric layer isan approximately 0.71 μm thick BST layer and is matched with a 0.49 μmgold top electrode and a 0.56 μm platinum bottom electrode.
 11. A methodof modeling electrostrictive effects and acoustic resonances in atunable capacitor, comprising: adjusting empirically the characteristicimpedances and complex propagation constants to account for actualprocess variations in manufacturing of said tunable capacitor; adjustingempirically the characteristic impedances and complex propagationconstants to account for end-effects in the directions transversal tothe wave direction; modeling the electrostrictive effect by dividing aBST layer of said tunable capacitor into thin layers or slices with eachslice's thickness representing a small fraction of an acousticwavelength and injecting at each junction between slices, an acoustic“current” proportional to the electrical current through said voltagetunable capacitor.
 12. The method of claim 11, wherein the real part ofthe vector sum of acoustic “voltages” at all slice junctions, divided bythe electrical current vector, is taken to be representative of thatpart of the voltage tunable capacitor's effective series resistancecontributed by the electrostrictive effect and acoustic resonances. 13.The method of claim 11, wherein said electrostrictive effect is thetransducer mechanism that links the electrical and acoustic domains. 14.A method of producing a high fundamental acoustic resonance frequency,comprising: placing a bottom electrode layer adjacent a substrate with avoltage tunable dielectric layer adjacent said bottom electrode layer,said voltage tunable dielectric layer including an active region;placing a top electrode adjacent said voltage tunable dielectric layerwith a final interconnect layer connected to said top electrode via aninterlayer; and using said top and bottom electrodes at a predeterminedthickness such that a desired high fundamental acoustic resonance isobtained.
 15. The method of claim 14, further comprising requiring saidactive region of said voltage tunable dielectric layer to beapproximately the length of said top electrode.
 16. The method of claim14, wherein said interlayer and said final interconnect layer cover onlya small fraction of said active region of said voltage tunabledielectric layer, thereby reducing the amplitude of resonances due tothe interlayer or final interconnect layer.
 17. The method of claim 14,further comprising using a substrate chosen to have a high acoustic lossfactor thereby reducing the amplitude of resonances due to the substratelayer.
 18. The method of claim 14, further comprising forming thethickness of said voltage tunable dielectric layer to a 300 nm thick BSTlayer and matching in with 150 nm gold top electrode and approximately200 nm platinum bottom electrode and wherein said interlayer and finalinterconnect layers cover only a small percentage of said active region.19. The apparatus of claim 1, wherein said voltage tunable dielectriclayer is a BST layer.
 20. (canceled)